fix(proxy): route pilot-agent HTTP via PilotTunnelBridge loopback (#989)

The remote-node HTTP proxy resolved targets by reading nodes.api_url and
nodes.api_token directly from the database. Both fields are empty for
pilot-agent nodes by design, which produced a misleading 503 ("no API URL
or token configured. Update it in Settings, Nodes.") for any API call
targeting a healthy pilot-agent: stack creation, log retrieval, and every
other resource a pilot-agent should serve.

NodeRegistry.getProxyTarget already encapsulates the correct dispatch.
For proxy mode it returns the persisted api_url and api_token. For
pilot-agent it returns the loopback URL of the active PilotTunnelBridge
with an empty token, since the bridge re-authenticates implicitly via the
pre-verified tunnel socket.

Switch all three lookup sites in remoteNodeProxy to this helper, cache
the resolved target on req.proxyTarget so the http-proxy router and
proxyReq callbacks do not re-resolve, and split the 503 message so
pilot-agent operators see "Pilot tunnel to X is disconnected" instead of
the proxy-mode hint.

backend/src/types/express.ts

@@ -19,6 +19,8 @@ declare global {
       mfaPendingUserId?: number;
       /** True when the pending MFA session originated from an SSO login (LDAP or OIDC) rather than a password login. */
       mfaPendingSso?: boolean;
+      /** Cached remote-proxy target resolved by `remoteNodeProxy`'s outer gate so the http-proxy router/proxyReq callbacks do not re-resolve. */
+      proxyTarget?: { apiUrl: string; apiToken: string };
     }
   }
 }